restore margins on thumbnails, add comments about it's changes and how responsive affects it

less/thumbnails.less

@@ -1,5 +1,6 @@
 // THUMBNAILS
 // ----------
+// Note: `.thumbnails` and `.thumbnails > li` are overriden in responsive files
 
 // Make wrapper ul behave like the grid
 .thumbnails {
@@ -16,6 +17,7 @@
 .thumbnails > li {
   float: left; // Explicity set the float since we don't require .span* classes
   margin-bottom: @baseLineHeight;
+  margin-left: @gridGutterWidth;
 }
 
 // The actual thumbnail (can be `a` or `div`)